(CNN) -- U.S. and NATO airstrikes in Afghanistan nearly tripled between 2006 and 2007, fueling a public backlash, according to a report from a humanitarian watchdog.

An Afghan man prepares a grave after a U.S. airstrike in Afghanistan's Herat province last month.

The report, issued by Human Rights Watch Monday, also cites the Taliban's use of human shields and its deployment of fighters "in populated villages, at times with the specific intent to shield their forces from counterattack."

The report comes as the U.S. military probes an August 22 airstrike that Afghan government officials say killed 90 civilians. The U.S. military says the strike killed five to seven civilians.

Airstrikes killed 116 civilians in 2006 and 321 in 2007. This year, 119 have been killed so far: the figures do not include the August 22 incident, where the number of those killed is under dispute, the report said.

The report added that "twice as many tons of bombs were dropped in 2007 than in 2006" and that in June and July of 2008, the U.S. "dropped approximately as much as it did in all of 2006."

"While airstrikes typically drop in number during the winter due to decreased fighting, this was not the case during the winter of 2007-2008. December 2007 saw twice the number of airstrikes as July 2007.

"There has been a massive and unprecedented surge in the use of airpower in Afghanistan in 2008. In response to increased insurgent activity, twice as many tons of bombs were dropped in 2007 than in 2006," the report said.

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The group said almost all airstrike civilian deaths have occurred during "rapid-response strikes, often carried out in support of ground troops after they came under insurgent attacks."

"Rapid response airstrikes have meant higher civilian casualties, while every bomb dropped in populated areas amplifies the chance of a mistake," said Brad Adams, Asia director at Human Rights Watch.

"Mistakes by the U.S. and NATO have dramatically decreased public support for the Afghan government and the presence of international forces providing security to Afghans."

Human Rights Watch also cites what it calls "the poor response" by U.S. officials when such deaths occur. It says U.S. officials "often immediately deny responsibility for civilian deaths or place all blame on the Taliban."

"U.S. investigations conducted have been unilateral, ponderous, and lacking in transparency, undercutting rather than improving relations with local populations and the Afghan government."

The report also said a condolence payment system has not offered "timely and adequate compensation to assist civilians harmed by U.S. actions."

The report said the United States and NATO announced changes in tactics that led to a drop in the casualties at the end of 2007. One of those was "delaying attacks where civilians might be harmed."

"However, alarmingly, civilian deaths are once again climbing," the report said.

The report said the problem will continue "without improvements in planning, intelligence, targeting, and identifying civilian populations."

"The U.S. needs to end the mistakes that are killing so many civilians," said Adams. "The U.S. must also take responsibility, including by providing timely compensation, when its airstrikes kill Afghan civilians.

"While Taliban shielding is a factor in some civilian deaths, the U.S. shouldn't use this as an excuse when it could have taken better precautions. It is, after all, its bombs that are doing the killing."

Brig. Gen. Richard Blanchette, spokesman for NATO's International Security Assistance Forces, said he thinks the report was "quite balanced" from an ISAF perspective since it accurately describes what the NATO and Operation Enduring Freedom commands have been doing.

Blanchette underscored one of the points the report made -- that Taliban militants are using the tactic of melting into the population when they are being chased, a move that exacerbates the civilian casualty problem.

The spokesman said ISAF regrets any civilian casualty and is always attempting to improve its tactics and procedures, such as providing effective communication between troops as air power is called in.